Previously, prisms or photonic elements were constructed using ordinary dielectric materials having common optical characteristics. Small or ultra-compact spectrometer constructed using these ordinary dielectric materials were limited regarding the amount of dispersion that was possible due to the reduced physical dimensions of the ordinary dielectric device.
This limited dispersion capability had the effect that only wavelengths within a relatively narrow range were detectable since the physical size limitation prevented wider dispersion. Further, even if wider dispersion was available, the relatively wide band of frequencies produced still lacked narrow band filtering to detect small differences in wavelengths. Thus, there remains a need for strong dispersion in an ultra compact spectrometer along with narrow band filtering in order to detect particular wavelength components present within a polychromatic wave.
A photonic crystal is a periodic structure consisting typically of two dielectric materials with high dielectric (k) contrast such as that between a semiconductor and air, for example, and with geometrical feature sizes comparable to or smaller than light wavelengths of interest. As an engineered structure or artificially engineered material, a photonic crystal can exhibit optical properties not commonly found in natural substances.
Properly designed photonic crystals can have color dispersion capabilities over certain prescribed frequency ranges that are one or two orders of magnitude stronger than those of ordinary dielectric materials. Further, extensive research has led to the discovery of several classes of photonic crystal structures for which the propagation of electromagnetic radiation is forbidden in certain frequency ranges. These forbidden frequency ranges can be called “photonic band gaps” (PBGs).
More recently, it has also been realized that electromagnetic radiation with frequencies adjacent to the boundaries of the photonic bands themselves can propagate quite differently than they would in ordinary optical materials, and can exhibit much stronger color dispersions via what can be called superprism effects.